Several systems and methods exist for identifying the position of a user and for performing surveying tasks in which the position of a mobile station has to be determined to varying degrees of accuracy.
One such system makes use of Global Positioning System (GPS) to determine the location of a mobile station. However, GPS has limited accuracy. As a result, systems have been developed to increase GPS accuracy by establishing a reference and identifying the degree of error, which is then used to correct the readings of other GPS systems. This is referred to as Differential GPS (DGPS). Real Time Kinematic (RTK) precision survey refers to Differential Global Positioning System (DGPS) applications that require centimeter level accuracy. The user establishes a base or reference station, and since the position of the base station can be accurately determined, it can be used to correct errors in GPS signals received at the base station. This error information is then relayed by radio (wireless data link) to a mobile station to provide correction data for GPS signals received by the mobile station. In this way the level of accuracy provided by the mobile GPS is greatly enhanced. Various considerations play into the accuracy of the correction information and versatility of the system, including the latency of the message (how often correction updates are sent) and range of the transmission. Different radio transmission techniques exist, including spread spectrum and narrow band, which display different advantages and disadvantages. In one system, for example, correction information is transmitted once a second in order to reduce the latency of the messages. FIG. 1 illustrates a typical Global Positioning System (GPS) used for surveying. The Global Positioning System (GPS) antenna 100 is mounted on the end of a hand-held survey pole 102. The remaining components of the system are contained in pockets within the backpack 104. For better range performance the radio antenna 106 is elevated to improve the line of sight with the transmitter at the base station (not shown). The antenna 106 is connected to the radio 108 with coaxial cable 110. A user interface device 112 is usually hand held or mounted on the side of the survey pole 102 with a serial interface cable 114 connecting it to the Global Positioning System (GPS) receiver 116. A battery 118 is also contained in the backpack 104 that provides power to the radio 108, Global Positioning System (GPS) receiver 116, the Global Positioning System (GPS) antenna 100, and sometimes also the user interface device 112. In this system the surveyor typically removes the battery 118 from the backpack every night and charges it with an external battery charger (not shown). FIG. 1 illustrates that this conventional system employs many different cables, each of which represents a potential failure point.
FIG. 2 illustrates a more integrated system, the model 4800, that is currently manufactured by Trimble Navigation Limited, of Sunnyvale, Calif. This implementation has integrated into a single enclosure the Global Positioning System (GPS) antenna 200, the Global Positioning System (GPS) receiver (not shown), the radio antenna (not shown), and the radio (not shown). The user interface device 210 remains externally connected to the Global Positioning System (GPS) receiver through an external cable 212. This system is powered with a battery (not shown) that is located at the bottom of the survey pole 214. Power is conducted upwards to the rest of the system through two insulated wires (not shown) that run through the center of the hollow survey pole 214. The top of the survey pole 214 and the electronics housing 220 are complementarily threaded to provide a mechanical connector. The threaded connector contains two concentric electrical contacts that allow the electrical circuit to be completed when the threaded top of the pole engages with the housing 220. Because the electrical contacts are concentric, the circuit is made independent of the final angular orientation of the pole as it is tightened. Power to the user interface device 210 that is clamped on to the side of the pole is transmitted from the battery in the bottom of the pole 214, up to the housing 220, and back down through the external cable 212. When the battery requires charging, the pole is disconnected from the electrical housing and inserted upside down into a separate charging fixture (not shown) that makes electrical contact with electrical contacts in the pole.
In order to further enhance the reliability of the system, it would be desirable to eliminate all external cables. Furthermore, the Trimble system integrally combines the wireless data link (radio) and corresponding antenna, with the Global Positioning System (GPS) receiver and its antenna. This limits the versatility of the system, since different wireless data link technologies are appropriate for different survey activities as will be further explained below. For instance, radio technologies are legislated differently from country to country whereas the Global Positioning System (GPS) is accessible worldwide.
Another surveying system is known as Geographic Information System (GIS) mapping. This is a survey activity that requires accuracy of the order of three meters. These systems also make use of Differential Global Positioning System (DGPS) corrections but they typically make use of publicly available broadcasts such as those made by the United States Coast Guard (USCG) in accordance with the IALA standard using Radio Navigation Beacon (RNB) transmitters. This obviates the requirement for the user to set up a reference station transmitter. The USCG has distributed these transmitters in an overlapping coverage pattern on all coastal and major inland waterways in the US, and is presently in the process of expanding this coverage to include the United States inland landmass. In fact, RNB transmitters are distributed pretty widely internationally. The Radio Navigation Beacon (RNB) transmissions are optimized for range in favor of latency and consequently employ data rates of 200 bits per second on carrier frequencies in the 300 kHz range. The mobile stations of Geographic Information System (GIS) users are equipped with Global Positioning System (GPS) and Radio Navigation Beacon (RNB) receivers to enable them to measure their positions. The surveyor typically mounts the Global Positioning System (GPS) antenna at the top of a survey pole, while the user interface device is either hand held or clamped to the side of the pole. The RNB receiver is typically carried in a backpack (very similar to the arrangement shown in FIG. 1) and the RNB antenna is usually mounted on a separate pole that is secured to the frame of the backpack.
Yet another approach for performing surveying involves the use of Robotic Total Station (RTS) surveyors. Instead of GPS, the mobile stations determine their position relative to a base station by means of laser beams and prismatic reflectors to make precise elevation, angle, and range measurements. The typical separation between the total station (laser head at the base station) and the measurement point (prismatic reflector at the mobile station) is typically 1000 meters or less. The total station is mounted on a stationary tripod and uses interferometry to determine the linear distance to the prismatic reflector. The total station has actuators that can aim the laser and transducers that can measure the corresponding pitch and yaw motion of the laser. The prismatic reflector of the mobile station is typically mounted on the end of a survey pole, remotely located at the point to be measured. The total station has the ability to lock onto the prism, and track it, as it moves to new locations. A two-way, wireless data link is employed between the reflector and the total station to allow the location data of the survey point (calculated at the total station end) to be communicated to the surveyor holding the prismatic reflector at the survey point (at the mobile station). For this application, the wireless data link is optimized for higher data rates and may sacrifice range. The communication path is always line of sight (as required for the laser system) and frequently employs spread spectrum wireless technology. A user interface device is either hand held or clamped to the side of the survey pole that supports the reflector.